Research On The Simulation And Detection Mechanism Of Multiple Scattering Signal Of Spaceborne Oceanic Lidar

The ocean subsurface is the main aera for plankton distribution and atmosphere-ocean energy exchange,which plays a vital role in the marine ecosystems,the ocean material cycle and global climate change.The optical properties of the subsurface ocean are closely related to components in the ocean,such as plankton,nutrients and particulate matters,etc.Detection of the optical properties of the subsurface ocean with high accuracy is particularly important for studying the marine ecosystems and understanding the role of the ocean in climate change.The spaceborne oceanic lidar is one of the most attractive techniques in ocean remote sensing.As an active remote sensing method,it has several irreplaceable advantages over in situ observation and ocean color method,such as the ability to obtain information about the vertical structure of ocean and observation during all days and nights without limitation of the solar zenith angle.At present,the spaceborne oceanic lidar systems are in proposal or under development.Research of the forward simulation model and detection mechanism in the early stage is of great significance for the design of a spaceborne oceanic lidar hardware system with good performance and making better use of its detetion capabilities.Based on the radiative transfer forward model,the detection mechanism,retrieval algorithm and hardware parameters of the spaceborne oceanic lidar are explored.The main research contents of this dissertation are as follows:A numerical simulation model of the spaceborne oceanic lidar returns is established by using Monte Carlo(MC)method.In order to understand the spaceborne oceanic lidar returns directly,a semianalytic MC lidar radiative transfer model is built for vertically inhomogeneous ocean.The validation is carried out via stochastic phenomenology method and it shows that this simulation model is an effective tool for exploring the detection mechanism of lidar.The influence of the multiple scattering effect on the spaceborne oceanic lidar return is analyzed.The depth dependence of lidar effective attenuation coefficient and the relationships between the lidar effective attenuation coefficient and the Inherent Optical Properties(IOPs)of seawater are first proposed.The depth dependence of lidar effective attenuation coefficient in homogeneous waters is discovered and explained through the simulations and analysis of the spaceborne lidar returns from several case 1 waters,and an exponential relationship between effective attenuation coefficient and depth is proposed.Furthermore,based on the influnce of seawater's IOPs on multiple scattering,the specific relationship among the effective attenuation coefficient,depth and IOPs of seawater is proposed,with accuracy verified in inhomogeneous case 1 waters.This relationship can improve the retrieval accuracy of the lidar attenuation coefficient by an order of magnitude.A MC simulation model for polarized oceanic lidar signal is established,and the effects of multiple scattering on the polarization characteristics of the lidar returns is analyzed by using this model.A semianalytic Monte Carlo polarized radiative transfer model is developed using the meridian planes method.In order to verify the validity of the model in terms of lidar returns and depolarization ratio thoroughly,a shipborne polarized lidar system is built and field experiments are carried out in the Yellow Sea of China.This simulation model is also used to quantitatively analyze the influence of multiple scattering on the polarization characteristics of the spaceborne oceanic lidar returns,which lays a theoretical basis for studying the detection mechanism and developing the retrieval model of the polarization lidar.The influence of atmosphere-ocean interface which is inevitable in the spaceborne oceanic lidar radiative transfer is discussed.The forward radiative transfer model of the spaceborne lidar is completed with modeling the wind-roughed sea surface.The influence of the wind-roughed sea surface on the laser transmission direction and the accuracy of the depth information in the lidar returns are analyzed comprehensively via the MC method.Moreover,the incident angle of lidar that should be chosen to avoid the strong reflections by the sea surface is recommended.The characteristics of bubbles in the upper ocean due to wind-driven sea wave breaking and biological activities are analyzed.The influence of bubbles in the water on the polarization characteristics of the lidar returns is discussed.Furthermore,the bubble concentration in the open ocean water that cannot be ignored is given.Finally,the maximum detection depths and optimum detection wavelengths of the spaceborne oceanic lidar influenced by the multiple scattering effect for global ocean detection are evaluated.The optical characteristic parameters of the global ocean from MODIS are used to calculate the signal-to-noise ratio(SNR)of the photon-counting spaceborne lidar.The maximum detection depths and the corresponding optimum wavelengths are calculated according to the limitation of SNR.Furthermore,taking advantage of the characteristics of the Fraunhofer absorption lines in the optical spectrum of the sun,the improvements in the SNR and the detection depth are analyzed.The advantage of setting the central laser wavelength of spaceborne oceanic lidar to match with H-? absorption line is proposed.This result provides theoretical guidance for devolepment of spaceborne oceanic lidar system with better detection performance.